High affinity, stereospecific recognition sites (receptors) for neurotransmitters, neuromodulators, and many therapeutic agents have been identified in both the central nervous system and peripheral tissues. It is currently thought that the interaction of a neurotransmitter, neuromodulator, or drug with these sites will initiate a series of events resulting in either a physiologic/behavioral response (in the case of a neurotransmitter or neuromodulator) or a pharmacologic action (in the case of a drug). High affinity and stereoselective binding of therapeutic agents to tissues also suggests that novel endogenous substances may be present which physilogically mimic (or antagonize) the actions of these therapeutic agents. Several receptor-effector systems are currently under study, including: a) the benzodiazepine-GABA receptor chloride ionophore complex. It is now evident that benzodiazepine receptors in the central nervous system are physically coupled to a subpopulation of GABA/A receptors, and that both benzodiazepine receptors and GABA receptors are functionally linked to a chloride ionophore. Thus the entire "supramolecular complex" is currently under study. b) "Peripheral-type" benzodiazepine receptors, which are present in both peripheral tissues and the central nervous system, but are physically and pharmacologically distinct from (a). c) Receptors for central stimulants (e.g., amphetamine, methylphenidate). d) Receptors for hallucinogens (e.g., phencyclidine), and e) Recognition sites for agents that regulate voltage sensitive calcium channels (e.g., dihydropyridines).